Consider a porous medium in which a species diffuses throughout the pore space with diffusion coefficient \({D}\)  and can react at the pore-solid interface via a surface with reaction rate \(\kappa \) . The diffusion-controlled limit is obtained when \(\kappa \rightarrow \infty \) , while taking \(\kappa \rightarrow 0\)  corresponds to a perfectly reflective interface. A quantity of central interest in such diffusion and reaction problems is the mean survival time \(\tau \) , which is the average lifetime of the diffusing species before it gets trapped. Another important quantity, which is also pertinent to the description of viscous flow in porous media [1]}, is the principal relaxation time \(T_1\)  associated with the time-dependent decay of the initially uniform concentration field of the diffusing particles [2]}.
